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83 lines
3.9 KiB
83 lines
3.9 KiB
HRTF Support
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============
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Starting with OpenAL Soft 1.14, HRTFs can be used to enable enhanced
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spatialization for both 3D (mono) and multi-channel sources, when used with
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headphones/stereo output. This can be enabled using the 'hrtf' config option.
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For multi-channel sources this creates a virtual speaker effect, making it
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sound as if speakers provide a discrete position for each channel around the
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listener. For mono sources this provides much more versatility in the perceived
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placement of sounds, making it seem as though they are coming from all around,
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including above and below the listener, instead of just to the front, back, and
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sides.
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The default data set is based on the KEMAR HRTF data provided by MIT, which can
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be found at <http://sound.media.mit.edu/resources/KEMAR.html>.
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Custom HRTF Data Sets
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=====================
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OpenAL Soft also provides an option to use user-specified data sets, in
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addition to or in place of the default set. This allows users to provide data
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sets that could be better suited for their heads, or to work with stereo
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speakers instead of headphones, for example.
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The file format is specified below. It uses little-endian byte order.
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==
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ALchar magic[8] = "MinPHR03";
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ALuint sampleRate;
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ALubyte channelType; /* Can be 0 (mono) or 1 (stereo). */
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ALubyte hrirSize; /* Can be 8 to 128 in steps of 8. */
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ALubyte fdCount; /* Can be 1 to 16. */
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struct {
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ALushort distance; /* Can be 50mm to 2500mm. */
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ALubyte evCount; /* Can be 5 to 128. */
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ALubyte azCount[evCount]; /* Each can be 1 to 128. */
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} fields[fdCount];
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/* NOTE: ALbyte3 is a packed 24-bit sample type,
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* hrirCount is the sum of all azCounts.
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* channels can be 1 (mono) or 2 (stereo) depending on channelType.
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*/
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ALbyte3 coefficients[hrirCount][hrirSize][channels];
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ALubyte delays[hrirCount][channels]; /* Each can be 0 to 63. */
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==
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The data layout is as follows:
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The file first starts with the 8-byte marker, "MinPHR03", to identify it as an
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HRTF data set. This is followed by an unsigned 32-bit integer, specifying the
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sample rate the data set is designed for (OpenAL Soft will resample the HRIRs
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if the output device's playback rate doesn't match).
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Afterward, an unsigned 8-bit integer specifies the channel type, which can be 0
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(mono, single-channel) or 1 (stereo, dual-channel). After this is another 8-bit
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integer which specifies how many sample points (or finite impulse response
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filter coefficients) make up each HRIR.
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The following unsigned 8-bit integer specifies the number of fields used by the
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data set, which must be in descending order (farthest first, closest last).
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Then for each field an unsigned 16-bit short specifies the distance for that
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field in millimeters, followed by an 8-bit integer for the number of
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elevations. These elevations start at the bottom (-90 degrees), and increment
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upwards. Following this is an array of unsigned 8-bit integers, one for each
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elevation which specifies the number of azimuths (and thus HRIRs) that make up
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each elevation. Azimuths start clockwise from the front, constructing a full
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circle. Mono HRTFs use the same HRIRs for both ears by reversing the azimuth
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calculation (ie. left = angle, right = 360-angle).
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The actual coefficients follow. Each coefficient is a signed 24-bit sample.
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Stereo HRTFs interleave left/right ear coefficients. The HRIRs must be
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minimum-phase. This allows the use of a smaller filter length, reducing
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computation. For reference, the default data set uses a 32-point filter while
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even the smallest data set provided by MIT used a 128-sample filter (a 4x
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reduction by applying minimum-phase reconstruction).
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After the coefficients is an array of unsigned 8-bit delay values as 6.2 fixed-
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point integers, one for each HRIR (with stereo HRTFs interleaving left/right
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ear delays). This is the propagation delay in samples a signal must wait before
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being convolved with the corresponding minimum-phase HRIR filter.
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